Evolution of the semiconductor manufacturing industry is placing ever greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions are shrinking while wafer size is increasing. Economics is driving the industry to decrease the time for achieving high-yield, high-value production and better device performance. Thus, minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for the semiconductor manufacturer.
To control device performance, in particular electrostatic characteristics, the critical dimensions (CD) of a device and thin film thicknesses are monitored. The gate dielectric, high-k, and work function materials and composition affect the device performance. This can impact, for example, the threshold voltage and the drive current. At the 10 nm node and below, some critical film thickness process windows can be below 0.3 A. Post-layer effect on pre-layer process is another issue that may need to be controlled. The metal gate deposition can alter the materials properties and their composition through annealing or diffusion in addition to potentially causing interfacial and crystalline defects. Therefore, it is becoming critical to measure every process step during formation of high-k metal gate stacks.
The high-k metal gate film formation process is complex. Typically, it can include six to nine film stacks depending on the technology node, application, and the device type (e.g., NMOS or PMOS).
In an example, after dummy-gate removal and oxide strip and clean, a series of ultra-thin materials are deposited on the gate prior to the deposition of the metal. Those materials are typically very thin and their optical dispersion properties are similar. In this example, the process starts with the deposition of an 8 A to 10 A SiO2 interface layer (IL) on the gate (and fin). This can be followed by a 14 A hafnium-dioxide as high-k material. This is followed by 10 A TiN work function material, then a barrier metal TaN (5 A to 10 A), then another TiN layer, then a TiAlC layer, and then a 10 A TiN layer. The depositions can be formed through atomic layer deposition (ALD). PMOS and NMOS requires different high-k metal gate (HKMG) process flows, which can change the number of critical layers. The HKMG process is at the end of the front end of the line (FEOL) loop of the CMOS device.
Traditionally, these films were measured on a planar film stacks pad. X-ray metrology was used to measure the multi-layer stack on planar, but x-ray was insufficient because of the required precision 3 sigma (<0.04 A) and because of its insufficient sensitivity due to little scattering of the thin films.
For the optical metrology, the complex stacks induce highly correlated parameters. Fixing some thicknesses at a nominal value may be necessary to reduce the correlation. But this method is not a satisfactory solution for at least three reasons. First, the pre-layers process may not be stable enough to assign one fixed value for that parameter. Second, any process step can change the properties of the pre-layer. Third, throughput may be too slow for commercial manufacturing because multiple layers cannot be measured in one step.
Starting at the 10 nm technology node, semiconductor manufacturers have sought to measure some critical film thickness on grating (2D and 3D). The correlation of film thicknesses and/or properties measured on the 1D film pads to real device characteristics such as WAT (Wafer acceptance test like Vth) is degrading. Specifically, for some FinFET layers, 1D proxy film targets have limited correlation to real process variation on the real FinFET transistor. Due to the loading effect where the deposition and etch rates are topography dependent, the 1D film data will not be in perfect correlation with the 2D or 3D ones. Previous methods were used to perform these measurements, but both are either incapable of providing accurate measurements and/or too slow for commercial manufacturing. These previous methods are known as the “single angle of incidence (AOI) all floating” method and the “Data Feedforward” method.
The single AOI all floating method uses a single AOI spectroscopic ellipsometry spectrum or single AOI rotating polarizer rotating compensator (RPRC) spectrum and floats all critical parameters and degrees of freedom simultaneously. This method cannot solve most of the issues and the requirements described above. The all floating method also cannot deal with low contrast or thin materials. One reason is the similarity in the optical properties of SiO2 and HfO2 and of TiN and TaN. This leads to correlation between concurrent parameters and, therefore, inaccurate measured film on grating thicknesses. For example, see H. Chouaib and Q. Zhao, “Nanoscale optical critical dimension measurement of a contact hole using deep ultraviolet spectroscopic ellipsometry”, J. Vac. Sci. Technol. B 31, 011803 (2013), which is incorporated by reference in its entirety. In an experiment, both simulation data and real experimental data obtained from the single AOI all floating method did not pass precision, accuracy, robustness, or wafer consistency tests. While the single AOI all floating method can potentially meet the throughput and cost of ownership (COO) requirements, it failed in most technical checks.
Below are examples of single AOI all floating method results. This example is a theoretical simulation of the expected parameters sensitivity, correlation, and precision. For this example, simulations are performed on fourteen HKMG layers. Only three layers are presented here. In the cap (TiN), the film stack is IL(SiO2)/High-k(HfO2)/TiN. In this structure, a total of eight geometrical parameters are floated simultaneously (FIG. 2).
The simulation shows that the expected precision 3 sigma of the three films IL, HK and Cap are 0.13 A, 0.13 A and 0.06 A, respectively. Semiconductor manufacturer specifications for precision may be as small as 0.03 A. This method fell short of the specifications in terms of precision. Also, parameters correlation indexes are 0.958, 0.958 and 0.846, respectively. For correlation, on the scale of zero to one when one represents a 100% parameter correlation, 0.958 and 0.846 correlations are considered high and represent a potential risk of the combined model and technique used. In addition to the simulation, the single AOI all floating method experimental data failed the robustness test and the precision test for this layer.
In the TaN layer, the film stack is IL(SiO2)/High-k(HfO2)/TiN/TaN. In this structure (FIG. 3), a total of nine parameters are floated simultaneously. The simulation showed poor precision results and high parameter correlation which do not meet the semiconductor manufacturer requirements for this layer. Experimental data also showed results that are out of specification.
In the N Metal Gate (NMG) deposition layer, the film stack complexity increases at the late stage of the HKMG process. The layer comprises several pre-layer stacks that affect the critical parameters measurement on grating. There are three critical parameters for this layer: the TaN (10 A), the TaL (40 A), and the TiN (8 A). These three layers must be measured simultaneously for two reasons. First, the TaN undergoes some treatment (recessed) and the wafer will go to TaL and TiN deposition within the same chamber. This prevents the wafer from going out of the chamber for metrology step. The TaL and TiN are deposited in-situ. Hence the need for the tri-layer measurements using one recipe. The simulation data expects bad precision data and highly correlated parameters between all six film stacks. Thus, the single AOI all floating method cannot provide accurate measurements of this layer.
Data feedforward (DFF) refers to taking data sets on different pre-layers and passing common parameters forward to subsequent layers. For example, see Mihardja et al., Proc. SPIE 8324, Metrology, Inspection, and Process Control for Microlithography XXVI, 83241H (Mar. 29, 2012), which is incorporated by reference in its entirety. In other words, it refers to measuring each single step (pre-layer) and feedforwarding the data to the next step. This method is supposed to break the correlation between different film stacks. For instance, in previous steps it was possible to measure IL thickness and then feed forward the measurement results to the HK module when the HK film is to be measured. Similarly, it was possible to feedforward the HK value measured to the gate work function module. This method assumes that the pre-layers IL and HK are unchanged in terms of properties. However, this assumption is not valid for the current advanced technological nodes. DFF suffers from multiple drawbacks. First, the DFF throughput is too slow for semiconductor manufacturers. To use feedforward, every single step needs to be measured. The total number of film stacks to be measured may be, for example, fourteen. The predefined specification for throughput is to achieve robust measurements on all fourteen film stacks within only nine recipes or less. In other words, multiple layers need to be measured simultaneously, which is not possible with DFF.
Second, DFF cannot be used for the NMG (FIG. 4) layer and PMG layer. The TaL and TiN are deposited in-situ, such as using ALD. Moreover, the underlayer TaN undergoes some treatment within the same chamber as the following TaL+TiN deposition step. DFF cannot be used with this particular process flow.
Third, DFF assumes that the optical dispersion of the materials is unchanged before deposition and after deposition. This assumption may be incorrect. There are multiple phenomena that affect pre-layer properties after a new process such as annealing effect due to change in temperature during post-layer etch/dep, stress and strain variations due to post-layer deposition, or the surface and interface effects' impact on the optical properties of each layer at the angstrom scale.
Fourth, any inaccuracy or error in pre-layer model is carried out to all post layers.
Fifth, any Library Boundary Hit (LBH) in an early pre-layer stage will interrupt all measurements in post layers.
Neither DFF nor the single AOI all floating method can provide solutions to the film on grating market due to one or more of inaccurate data out of specifications linearity to a reference method, poor precision and stability and matching, highly correlated critical and floating parameters, slow measurement (i.e., low throughput), high COO, inability to handle in-situ ALD process, high risk for LBH, and robustness test failures.
Therefore, an improved method and system for measuring thin films is needed.